netdev - Re: [PATCH net v2] xen-netback: Fix handling of skbs requiring too many slots

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Message-Id: <20140605.150952.1419101660779617906.davem@davemloft.net>
Date:	Thu, 05 Jun 2014 15:09:52 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	zoltan.kiss@...rix.com
Cc:	xen-devel@...ts.xenproject.org, ian.campbell@...rix.com,
	wei.liu2@...rix.com, paul.durrant@...rix.com, linux@...elenboom.it,
	netdev@...r.kernel.org, david.vrabel@...rix.com
Subject: Re: [PATCH net v2] xen-netback: Fix handling of skbs requiring too
 many slots

From: Zoltan Kiss <zoltan.kiss@...rix.com>
Date: Wed, 4 Jun 2014 19:58:51 +0100

> A recent commit (a02eb4 "xen-netback: worse-case estimate in xenvif_rx_action is
> underestimating") capped the slot estimation to MAX_SKB_FRAGS, but that triggers
> the next BUG_ON a few lines down, as the packet consumes more slots than
> estimated.
> This patch introduces full_coalesce on the skb callback buffer, which is used in
> start_new_rx_buffer() to decide whether netback needs coalescing more
> aggresively. By doing that, no packet should need more than
> (XEN_NETIF_MAX_TX_SIZE + 1) / PAGE_SIZE data slots (excluding the optional GSO
> slot, it doesn't carry data, therefore irrelevant in this case), as the provided
> buffers are fully utilized.
> 
> Signed-off-by: Zoltan Kiss <zoltan.kiss@...rix.com>

Applied, and:

> -		if (max_slots_needed > MAX_SKB_FRAGS)
> +		if (max_slots_needed > MAX_SKB_FRAGS) {
>  			max_slots_needed = MAX_SKB_FRAGS;
> +			XENVIF_RX_CB(skb)->full_coalesce = true;
> +		} else
> +			XENVIF_RX_CB(skb)->full_coalesce = false;

I took care of adding the {} to the else block, as suggested by Wei Liu.
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